Affine coding with offsets

ABSTRACT

In a method for video encoding in an encoder, a base predictor of a block in a current picture is determined. A plurality of offset indexes in a plurality of respective pre-defined mapping tables is determined, the plurality of offset indexes indicating corresponding offset values to be applied to parameters of an affine model for the base predictor. A coded video bitstream is generated, the coded video bitstream including prediction information that indicates the plurality of offset indexes. The pre-defined mapping tables include a first mapping table of distance offset indexes that are mapped to different pixel distances and a second mapping table of offset direction indexes that are mapped to different pairs of offset directions on an x-axis and a y-axis.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.17/242,506 filed Apr. 28, 2021, which is a continuation of U.S.application Ser. No. 16/398,308 filed Apr. 30, 2019, now U.S. Pat. No.11,039,157, which claims the benefit of priority to U.S. ProvisionalApplication No. 62/734,998 filed Sep. 21, 2018, the entire contents ofeach of which are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure describes embodiments generally related to videocoding.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent the work is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentdisclosure.

Video coding and decoding can be performed using inter-pictureprediction with motion compensation. Uncompressed digital video caninclude a series of pictures, each picture having a spatial dimensionof, for example, 1920×1080 luminance samples and associated chrominancesamples. The series of pictures can have a fixed or variable picturerate (informally also known as frame rate), of, for example 60 picturesper second or 60 Hz. Uncompressed video has significant bitraterequirements. For example, 1080p60 4:2:0 video at 8 bit per sample(1920×1080 luminance sample resolution at 60 Hz frame rate) requiresclose to 1.5 Gbit/s bandwidth. An hour of such video requires more than600 GBytes of storage space.

One purpose of video coding and decoding can be the reduction ofredundancy in the input video signal, through compression. Compressioncan help reduce the aforementioned bandwidth or storage spacerequirements, in some cases by two orders of magnitude or more. Bothlossless and lossy compression, as well as a combination thereof can beemployed. Lossless compression refers to techniques where an exact copyof the original signal can be reconstructed from the compressed originalsignal. When using lossy compression, the reconstructed signal may notbe identical to the original signal, but the distortion between originaland reconstructed signals is small enough to make the reconstructedsignal useful for the intended application. In the case of video, lossycompression is widely employed. The amount of distortion tolerateddepends on the application; for example, users of certain consumerstreaming applications may tolerate higher distortion than users oftelevision distribution applications. The compression ratio achievablecan reflect that: higher allowable/tolerable distortion can yield highercompression ratios.

Motion compensation can be a lossy compression technique and can relateto techniques where a block of sample data from a previouslyreconstructed picture or part thereof (reference picture), after beingspatially shifted in a direction indicated by a motion vector (MVhenceforth), is used for the prediction of a newly reconstructed pictureor picture part. In some cases, the reference picture can be the same asthe picture currently under reconstruction. MVs can have two dimensionsX and Y, or three dimensions, the third being an indication of thereference picture in use (the latter, indirectly, can be a timedimension).

In some video compression techniques, an MV applicable to a certain areaof sample data can be predicted from other MVs, for example from thoserelated to another area of sample data spatially adjacent to the areaunder reconstruction, and preceding that MV in decoding order. Doing socan substantially reduce the amount of data required for coding the MV,thereby removing redundancy and increasing compression. MV predictioncan work effectively, for example, because when coding an input videosignal derived from a camera (known as natural video) there is astatistical likelihood that areas larger than the area to which a singleMV is applicable move in a similar direction and, therefore, can in somecases be predicted using a similar motion vector derived from MVs ofneighboring area. That results in the MV found for a given area to besimilar or the same as the MV predicted from the surrounding MVs, andthat in turn can be represented, after entropy coding, in a smallernumber of bits than what would be used if coding the MV directly. Insome cases, MV prediction can be an example of lossless compression of asignal (namely: the MVs) derived from the original signal (namely: thesample stream). In other cases, MV prediction itself can be lossy, forexample because of rounding errors when calculating a predictor fromseveral surrounding MVs.

Various MV prediction mechanisms are described in H.265/HEVC (ITU-T Rec.H.265, “High Efficiency Video Coding”, December 2016). Out of the manyMV prediction mechanisms that H.265 offers, described here is atechnique henceforth referred to as “spatial merge”.

Referring to FIG. 1 , a current block (101) comprises samples that havebeen found by the encoder during the motion search process to bepredictable from a previous block of the same size that has beenspatially shifted. Instead of coding that MV directly, the MV can bederived from metadata associated with one or more reference pictures,for example from the most recent (in decoding order) reference picture,using the MV associated with either one of five surrounding samples,denoted A0, A1, and B0, B1, B2 (102 through 106, respectively). InH.265, the MV prediction can use predictors from the same referencepicture that the neighboring block is using.

SUMMARY

Aspects of the disclosure provide methods and apparatuses for videoencoding/decoding. In some examples, an apparatus for video decodingincludes receiving circuitry and processing circuitry.

According to an aspect of the disclosure, a method for video decoding ina decoder is provided. In the disclosed method, prediction informationof a block is decoded in a current picture from a coded video bitstream.The prediction information includes a plurality of offset indices forprediction offsets associated with an affine model in an interprediction mode. Subsequently, parameters of the affine model aredetermined based on the plurality of offset indices. Each of theplurality of the offset indices including a respective pre-definedmapping table that includes indexes and corresponding offset values. Theparameters of the affine model are used to transform between the blockand a reference block in a reference picture that has beenreconstructed. Further, at least a sample of the block is reconstructedaccording to the affine model.

In some embodiments, the plurality of offset indices include at leastone of a distance offset index, an offset direction index, a deltascaling index, and a delta rotation index.

In some embodiments, a base predictor of the block is determined from apredictor candidate list based on a base predictor index that issignaled. The block includes two or more control points, the predictorcandidate list including more than one predictor candidates. In someembodiments, and a base predictor of the block is determined based on apredefined base predictor index when the base predictor index is notsignaled.

In some embodiments, the distance offset index is decoded to determine adistance offset value based on the respective pre-determined mappingtable of the distance offset index. The offset direction index isdecoded to determine an offset direction based on the respectivepre-determined mapping table of the offset direction index. A motionvector for one of the two or more control points of the block issubsequently derived in the current picture based on at least one of thebase predictor, the distance offset value, and the offset direction.

In some embodiments, a motion vector of a control point of the basepredictor is set as the motion vector for the one of the two or morecontrol points of the block in the current picture based on adetermination that a zero motion vector difference flag is true.

In some embodiments, the distance offset value and the offset directionare applied onto a motion vector of a control point of the basepredictor to generate the motion vector for the one of the two or morecontrol points of the block in the current picture based on adetermination that a zero motion vector difference flag is false.

In some embodiments, a first distance offset value and a first offsetdirection are applied onto a first motion vector of a control point ofthe base predictor on a first inter prediction direction to generate afirst motion vector for the one of the two or more control points of theblock in the current picture on the first inter prediction directionbased on a determination that a first zero motion vector difference flagis false. In addition, a second distance offset value and a secondoffset direction are applied onto a second motion vector of the controlpoint of the base predictor on a second inter prediction direction togenerate a second motion vector for the one of the two or more controlpoints of the block in the current picture on the second interprediction direction based on the determination that a second zeromotion vector difference flag is false.

In some embodiments, the pre-defined mapping table is adjustable andreceived at one of a sequence level, a slice level, a tile level, a tilegroup level, and a block level.

In some embodiments, the delta scaling index is decoded to determine adelta scaling parameter based on the respective pre-determined mappingtable of the delta scaling index. The delta rotation index is decoded todetermine a delta rotation parameter based on the respectivepre-determined mapping table of the delta rotation index. The distanceoffset index is decoded to determine a distance offset value based onthe respective pre-determined mapping table of the distance offsetindex. The offset direction index is decoded to determine an offsetdirection based on the respective pre-determined mapping table of theoffset direction index. A motion vector for one of the two or morecontrol points of the block is subsequently derived in the currentpicture based on at least one of the base predictor, the delta scalingparameter, the delta rotation parameter, the distance offset value, andthe offset direction.

In some embodiments, a scaling parameter of the base predictor is set asa scaling parameter of the block in the current picture based on adetermination that a zero delta flag is true. The delta scalingparameter is applied to the scaling parameter of the base predictor togenerate the scaling parameter of the block based on a determinationthat the zero delta flag is false.

In some embodiments, a rotation parameter of the base predictor is setas a rotation parameter of the block based on a determination that azero delta flag is true. The delta rotation parameter is applied to therotation parameter of the base predictor to generate the rotationparameter of the block in the current picture based on a determinationthat the zero delta flag is false.

In some embodiments, a translational motion vector of the base predictoris set as a translational motion vector of the block based on adetermination that a zero motion vector difference flag is true. Thedistance offset value and the offset direction are applied onto thetranslational motion vector of the base predictor to generate thetranslational motion vector of the block based on a determination thatthe zero motion vector difference flag is false.

According to another aspect of the disclosure, an apparatus is provided.The apparatus has processing circuitry. The processing circuitry isconfigured to perform the disclosed method for video coding.

Aspects of the disclosure also provide a non-transitorycomputer-readable medium storing instructions which when executed by acomputer for video decoding cause the computer to perform the method forvideo decoding.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features, the nature, and various advantages of the disclosedsubject matter will be more apparent from the following detaileddescription and the accompanying drawings in which:

FIG. 1 is a schematic illustration of a current block and itssurrounding spatial merge candidates in one example.

FIG. 2 is a schematic illustration of a simplified block diagram of acommunication system (200) in accordance with an embodiment.

FIG. 3 is a schematic illustration of a simplified block diagram of acommunication system (300) in accordance with an embodiment.

FIG. 4 is a schematic illustration of a simplified block diagram of adecoder in accordance with an embodiment.

FIG. 5 is a schematic illustration of a simplified block diagram of anencoder in accordance with an embodiment.

FIG. 6 shows a block diagram of an encoder in accordance with anotherembodiment.

FIG. 7 shows a block diagram of a decoder in accordance with anotherembodiment.

FIG. 8 shows an example of spatial and temporal candidates in someexamples.

FIG. 9 shows examples for UMVE according to an embodiment of thedisclosure.

FIG. 10 shows examples for UMVE according to an embodiment of thedisclosure.

FIG. 11 shows an example of a block with an affine motion model.

FIG. 12 shows examples of affine transformation according to someembodiments of the disclosure.

FIG. 13 shows a diagram of a current block and two control points CP0and CP1 of the current block according to some embodiment of thedisclosure.

FIG. 14 shows a first flow chart outlining a process example accordingto some embodiments of the disclosure.

FIG. 15 shows a second flow chart outlining a process example accordingto some embodiments of the disclosure.

FIG. 16 shows a third flow chart outlining a process example accordingto some embodiments of the disclosure.

FIG. 17 is a schematic illustration of a computer system in accordancewith an embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 2 illustrates a simplified block diagram of a communication system(200) according to an embodiment of the present disclosure. Thecommunication system (200) includes a plurality of terminal devices thatcan communicate with each other, via, for example, a network (250). Forexample, the communication system (200) includes a first pair ofterminal devices (210) and (220) interconnected via the network (250).In the FIG. 2 example, the first pair of terminal devices (210) and(220) performs unidirectional transmission of data. For example, theterminal device (210) may code video data (e.g., a stream of videopictures that are captured by the terminal device (210)) fortransmission to the other terminal device (220) via the network (250).The encoded video data can be transmitted in the form of one or morecoded video bitstreams. The terminal device (220) may receive the codedvideo data from the network (250), decode the coded video data torecover the video pictures and display video pictures according to therecovered video data. Unidirectional data transmission may be common inmedia serving applications and the like.

In another example, the communication system (200) includes a secondpair of terminal devices (230) and (240) that performs bidirectionaltransmission of coded video data that may occur, for example, duringvideoconferencing. For bidirectional transmission of data, in anexample, each terminal device of the terminal devices (230) and (240)may code video data (e.g., a stream of video pictures that are capturedby the terminal device) for transmission to the other terminal device ofthe terminal devices (230) and (240) via the network (250). Eachterminal device of the terminal devices (230) and (240) also may receivethe coded video data transmitted by the other terminal device of theterminal devices (230) and (240), and may decode the coded video data torecover the video pictures and may display video pictures at anaccessible display device according to the recovered video data.

In the FIG. 2 example, the terminal devices (210), (220), (230) and(240) may be illustrated as servers, personal computers and smart phonesbut the principles of the present disclosure may be not so limited.Embodiments of the present disclosure find application with laptopcomputers, tablet computers, media players and/or dedicated videoconferencing equipment. The network (250) represents any number ofnetworks that convey coded video data among the terminal devices (210),(220), (230) and (240), including for example wireline (wired) and/orwireless communication networks. The communication network (250) mayexchange data in circuit-switched and/or packet-switched channels.Representative networks include telecommunications networks, local areanetworks, wide area networks and/or the Internet. For the purposes ofthe present discussion, the architecture and topology of the network(250) may be immaterial to the operation of the present disclosureunless explained herein below.

FIG. 3 illustrates, as an example for an application for the disclosedsubject matter, the placement of a video encoder and a video decoder ina streaming environment. The disclosed subject matter can be equallyapplicable to other video enabled applications, including, for example,video conferencing, digital TV, storing of compressed video on digitalmedia including CD, DVD, memory stick and the like, and so on.

A streaming system may include a capture subsystem (313), that caninclude a video source (301), for example a digital camera, creating forexample a stream of video pictures (302) that are uncompressed. In anexample, the stream of video pictures (302) includes samples that aretaken by the digital camera. The stream of video pictures (302),depicted as a bold line to emphasize a high data volume when compared toencoded video data (304) (or coded video bitstreams), can be processedby an electronic device (320) that includes a video encoder (303)coupled to the video source (301). The video encoder (303) can includehardware, software, or a combination thereof to enable or implementaspects of the disclosed subject matter as described in more detailbelow. The encoded video data (304) (or encoded video bitstream (304)),depicted as a thin line to emphasize the lower data volume when comparedto the stream of video pictures (302), can be stored on a streamingserver (305) for future use. One or more streaming client subsystems,such as client subsystems (306) and (308) in FIG. 3 can access thestreaming server (305) to retrieve copies (307) and (309) of the encodedvideo data (304). A client subsystem (306) can include a video decoder(310), for example, in an electronic device (330). The video decoder(310) decodes the incoming copy (307) of the encoded video data andcreates an outgoing stream of video pictures (311) that can be renderedon a display (312) (e.g., display screen) or other rendering device (notdepicted). In some streaming systems, the encoded video data (304),(307), and (309) (e.g., video bitstreams) can be encoded according tocertain video coding/compression standards. Examples of those standardsinclude ITU-T Recommendation H.265. In an example, a video codingstandard under development is informally known as Versatile Video Coding(VVC). The disclosed subject matter may be used in the context of VVC.

It is noted that the electronic devices (320) and (330) can includeother components (not shown). For example, the electronic device (320)can include a video decoder (not shown) and the electronic device (330)can include a video encoder (not shown) as well.

FIG. 4 shows a block diagram of a video decoder (410) according to anembodiment of the present disclosure. The video decoder (410) can beincluded in an electronic device (430). The electronic device (430) caninclude a receiver (431) (e.g., receiving circuitry). The video decoder(410) can be used in the place of the video decoder (310) in the FIG. 3example.

The receiver (431) may receive one or more coded video sequences to bedecoded by the video decoder (410); in the same or another embodiment,one coded video sequence at a time, where the decoding of each codedvideo sequence is independent from other coded video sequences. Thecoded video sequence may be received from a channel (401), which may bea hardware/software link to a storage device which stores the encodedvideo data. The receiver (431) may receive the encoded video data withother data, for example, coded audio data and/or ancillary data streams,that may be forwarded to their respective using entities (not depicted).The receiver (431) may separate the coded video sequence from the otherdata. To combat network jitter, a buffer memory (415) may be coupled inbetween the receiver (431) and an entropy decoder/parser (420) (“parser(420)” henceforth). In certain applications, the buffer memory (415) ispart of the video decoder (410). In others, it can be outside of thevideo decoder (410) (not depicted). In still others, there can be abuffer memory (not depicted) outside of the video decoder (410), forexample to combat network jitter, and in addition another buffer memory(415) inside the video decoder (410), for example to handle playouttiming. When the receiver (431) is receiving data from a store/forwarddevice of sufficient bandwidth and controllability, or from anisosynchronous network, the buffer memory (415) may not be needed, orcan be small. For use on best effort packet networks such as theInternet, the buffer memory (415) may be required, can be comparativelylarge and can be advantageously of adaptive size, and may at leastpartially be implemented in an operating system or similar elements (notdepicted) outside of the video decoder (410).

The video decoder (410) may include the parser (420) to reconstructsymbols (421) from the coded video sequence. Categories of those symbolsinclude information used to manage operation of the video decoder (410),and potentially information to control a rendering device such as arender device (412) (e.g., a display screen) that is not an integralpart of the electronic device (430) but can be coupled to the electronicdevice (430), as was shown in FIG. 4 . The control information for therendering device(s) may be in the form of Supplemental EnhancementInformation (SEI messages) or Video Usability Information (VUI)parameter set fragments (not depicted). The parser (420) mayparse/entropy-decode the coded video sequence that is received. Thecoding of the coded video sequence can be in accordance with a videocoding technology or standard, and can follow various principles,including variable length coding, Huffman coding, arithmetic coding withor without context sensitivity, and so forth. The parser (420) mayextract from the coded video sequence, a set of subgroup parameters forat least one of the subgroups of pixels in the video decoder, based uponat least one parameter corresponding to the group. Subgroups can includeGroups of Pictures (GOPs), pictures, tiles, slices, macroblocks, CodingUnits (CUs), blocks, Transform Units (TUs), Prediction Units (PUs) andso forth. The parser (420) may also extract from the coded videosequence information such as transform coefficients, quantizer parametervalues, motion vectors, and so forth.

The parser (420) may perform an entropy decoding/parsing operation onthe video sequence received from the buffer memory (415), so as tocreate symbols (421).

Reconstruction of the symbols (421) can involve multiple different unitsdepending on the type of the coded video picture or parts thereof (suchas: inter and intra picture, inter and intra block), and other factors.Which units are involved, and how, can be controlled by the subgroupcontrol information that was parsed from the coded video sequence by theparser (420). The flow of such subgroup control information between theparser (420) and the multiple units below is not depicted for clarity.

Beyond the functional blocks already mentioned, the video decoder (410)can be conceptually subdivided into a number of functional units asdescribed below. In a practical implementation operating undercommercial constraints, many of these units interact closely with eachother and can, at least partly, be integrated into each other. However,for the purpose of describing the disclosed subject matter, theconceptual subdivision into the functional units below is appropriate.

A first unit is the scaler/inverse transform unit (451). Thescaler/inverse transform unit (451) receives a quantized transformcoefficient as well as control information, including which transform touse, block size, quantization factor, quantization scaling matrices,etc. as symbol(s) (421) from the parser (420). The scaler/inversetransform unit (451) can output blocks comprising sample values, thatcan be input into aggregator (455).

In some cases, the output samples of the scaler/inverse transform (451)can pertain to an intra coded block; that is: a block that is not usingpredictive information from previously reconstructed pictures, but canuse predictive information from previously reconstructed parts of thecurrent picture. Such predictive information can be provided by an intrapicture prediction unit (452). In some cases, the intra pictureprediction unit (452) generates a block of the same size and shape ofthe block under reconstruction, using surrounding already reconstructedinformation fetched from the current picture buffer (458). The currentpicture buffer (458) buffers, for example, partly reconstructed currentpicture and/or fully reconstructed current picture. The aggregator(455), in some cases, adds, on a per sample basis, the predictioninformation the intra prediction unit (452) has generated to the outputsample information as provided by the scaler/inverse transform unit(451).

In other cases, the output samples of the scaler/inverse transform unit(451) can pertain to an inter coded, and potentially motion compensatedblock. In such a case, a motion compensation prediction unit (453) canaccess reference picture memory (457) to fetch samples used forprediction. After motion compensating the fetched samples in accordancewith the symbols (421) pertaining to the block, these samples can beadded by the aggregator (455) to the output of the scaler/inversetransform unit (451) (in this case called the residual samples orresidual signal) so as to generate output sample information. Theaddresses within the reference picture memory (457) from where themotion compensation prediction unit (453) fetches prediction samples canbe controlled by motion vectors, available to the motion compensationprediction unit (453) in the form of symbols (421) that can have, forexample X, Y, and reference picture components. Motion compensation alsocan include interpolation of sample values as fetched from the referencepicture memory (457) when sub-sample exact motion vectors are in use,motion vector prediction mechanisms, and so forth.

The output samples of the aggregator (455) can be subject to variousloop filtering techniques in the loop filter unit (456). Videocompression technologies can include in-loop filter technologies thatare controlled by parameters included in the coded video sequence (alsoreferred to as coded video bitstream) and made available to the loopfilter unit (456) as symbols (421) from the parser (420), but can alsobe responsive to meta-information obtained during the decoding ofprevious (in decoding order) parts of the coded picture or coded videosequence, as well as responsive to previously reconstructed andloop-filtered sample values.

The output of the loop filter unit (456) can be a sample stream that canbe output to the render device (412) as well as stored in the referencepicture memory (457) for use in future inter-picture prediction.

Certain coded pictures, once fully reconstructed, can be used asreference pictures for future prediction. For example, once a codedpicture corresponding to a current picture is fully reconstructed andthe coded picture has been identified as a reference picture (by, forexample, the parser (420)), the current picture buffer (458) can becomea part of the reference picture memory (457), and a fresh currentpicture buffer can be reallocated before commencing the reconstructionof the following coded picture.

The video decoder (410) may perform decoding operations according to apredetermined video compression technology in a standard, such as ITU-TRec. H.265. The coded video sequence may conform to a syntax specifiedby the video compression technology or standard being used, in the sensethat the coded video sequence adheres to both the syntax of the videocompression technology or standard and the profiles as documented in thevideo compression technology or standard. Specifically, a profile canselect certain tools as the only tools available for use under thatprofile from all the tools available in the video compression technologyor standard. Also necessary for compliance can be that the complexity ofthe coded video sequence is within bounds as defined by the level of thevideo compression technology or standard. In some cases, levels restrictthe maximum picture size, maximum frame rate, maximum reconstructionsample rate (measured in, for example megasamples per second), maximumreference picture size, and so on. Limits set by levels can, in somecases, be further restricted through Hypothetical Reference Decoder(HRD) specifications and metadata for HRD buffer management signaled inthe coded video sequence.

In an embodiment, the receiver (431) may receive additional (redundant)data with the encoded video. The additional data may be included as partof the coded video sequence(s). The additional data may be used by thevideo decoder (410) to properly decode the data and/or to moreaccurately reconstruct the original video data. Additional data can bein the form of, for example, temporal, spatial, or signal noise ratio(SNR) enhancement layers, redundant slices, redundant pictures, forwarderror correction codes, and so on.

FIG. 5 shows a block diagram of a video encoder (503) according to anembodiment of the present disclosure. The video encoder (503) isincluded in an electronic device (520). The electronic device (520)includes a transmitter (540) (e.g., transmitting circuitry). The videoencoder (503) can be used in the place of the video encoder (303) in theFIG. 3 example.

The video encoder (503) may receive video samples from a video source(501) (that is not part of the electronic device (520) in the FIG. 5example) that may capture video image(s) to be coded by the videoencoder (503). In another example, the video source (501) is a part ofthe electronic device (520).

The video source (501) may provide the source video sequence to be codedby the video encoder (503) in the form of a digital video sample streamthat can be of any suitable bit depth (for example: 8 bit, 10 bit, 12bit, . . . ), any colorspace (for example, BT.601 Y CrCB, RGB, . . . ),and any suitable sampling structure (for example Y CrCb 4:2:0, Y CrCb4:4:4). In a media serving system, the video source (501) may be astorage device storing previously prepared video. In a videoconferencingsystem, the video source (501) may be a camera that captures local imageinformation as a video sequence. Video data may be provided as aplurality of individual pictures that impart motion when viewed insequence. The pictures themselves may be organized as a spatial array ofpixels, wherein each pixel can comprise one or more samples depending onthe sampling structure, color space, etc. in use. A person skilled inthe art can readily understand the relationship between pixels andsamples. The description below focuses on samples.

According to an embodiment, the video encoder (503) may code andcompress the pictures of the source video sequence into a coded videosequence (543) in real time or under any other time constraints asrequired by the application. Enforcing appropriate coding speed is onefunction of a controller (550). In some embodiments, the controller(550) controls other functional units as described below and isfunctionally coupled to the other functional units. The coupling is notdepicted for clarity. Parameters set by the controller (550) can includerate control related parameters (picture skip, quantizer, lambda valueof rate-distortion optimization techniques, . . . ), picture size, groupof pictures (GOP) layout, maximum motion vector search range, and soforth. The controller (550) can be configured to have other suitablefunctions that pertain to the video encoder (503) optimized for acertain system design.

In some embodiments, the video encoder (503) is configured to operate ina coding loop. As an oversimplified description, in an example, thecoding loop can include a source coder (530) (e.g., responsible forcreating symbols, such as a symbol stream, based on an input picture tobe coded, and a reference picture(s)), and a (local) decoder (533)embedded in the video encoder (503). The decoder (533) reconstructs thesymbols to create the sample data in a similar manner as a (remote)decoder also would create (as any compression between symbols and codedvideo bitstream is lossless in the video compression technologiesconsidered in the disclosed subject matter). The reconstructed samplestream (sample data) is input to the reference picture memory (534). Asthe decoding of a symbol stream leads to bit-exact results independentof decoder location (local or remote), the content in the referencepicture memory (534) is also bit exact between the local encoder andremote encoder. In other words, the prediction part of an encoder “sees”as reference picture samples exactly the same sample values as a decoderwould “see” when using prediction during decoding. This fundamentalprinciple of reference picture synchronicity (and resulting drift, ifsynchronicity cannot be maintained, for example because of channelerrors) is used in some related arts as well.

The operation of the “local” decoder (533) can be the same as of a“remote” decoder, such as the video decoder (410), which has alreadybeen described in detail above in conjunction with FIG. 4 . Brieflyreferring also to FIG. 4 , however, as symbols are available andencoding/decoding of symbols to a coded video sequence by an entropycoder (545) and the parser (420) can be lossless, the entropy decodingparts of the video decoder (410), including the buffer memory (415), andparser (420) may not be fully implemented in the local decoder (533).

An observation that can be made at this point is that any decodertechnology except the parsing/entropy decoding that is present in adecoder also necessarily needs to be present, in substantially identicalfunctional form, in a corresponding encoder. For this reason, thedisclosed subject matter focuses on decoder operation. The descriptionof encoder technologies can be abbreviated as they are the inverse ofthe comprehensively described decoder technologies. Only in certainareas a more detail description is required and provided below.

During operation, in some examples, the source coder (530) may performmotion compensated predictive coding, which codes an input picturepredictively with reference to one or more previously-coded picture fromthe video sequence that were designated as “reference pictures”. In thismanner, the coding engine (532) codes differences between pixel blocksof an input picture and pixel blocks of reference picture(s) that may beselected as prediction reference(s) to the input picture.

The local video decoder (533) may decode coded video data of picturesthat may be designated as reference pictures, based on symbols createdby the source coder (530). Operations of the coding engine (532) mayadvantageously be lossy processes. When the coded video data may bedecoded at a video decoder (not shown in FIG. 5 ), the reconstructedvideo sequence typically may be a replica of the source video sequencewith some errors. The local video decoder (533) replicates decodingprocesses that may be performed by the video decoder on referencepictures and may cause reconstructed reference pictures to be stored inthe reference picture cache (534). In this manner, the video encoder(503) may store copies of reconstructed reference pictures locally thathave common content as the reconstructed reference pictures that will beobtained by a far-end video decoder (absent transmission errors).

The predictor (535) may perform prediction searches for the codingengine (532). That is, for a new picture to be coded, the predictor(535) may search the reference picture memory (534) for sample data (ascandidate reference pixel blocks) or certain metadata such as referencepicture motion vectors, block shapes, and so on, that may serve as anappropriate prediction reference for the new pictures. The predictor(535) may operate on a sample block-by-pixel block basis to findappropriate prediction references. In some cases, as determined bysearch results obtained by the predictor (535), an input picture mayhave prediction references drawn from multiple reference pictures storedin the reference picture memory (534).

The controller (550) may manage coding operations of the source coder(530), including, for example, setting of parameters and subgroupparameters used for encoding the video data.

Output of all aforementioned functional units may be subjected toentropy coding in the entropy coder (545). The entropy coder (545)translates the symbols as generated by the various functional units intoa coded video sequence, by lossless compressing the symbols according totechnologies such as Huffman coding, variable length coding, arithmeticcoding, and so forth.

The transmitter (540) may buffer the coded video sequence(s) as createdby the entropy coder (545) to prepare for transmission via acommunication channel (560), which may be a hardware/software link to astorage device which would store the encoded video data. The transmitter(540) may merge coded video data from the video coder (503) with otherdata to be transmitted, for example, coded audio data and/or ancillarydata streams (sources not shown).

The controller (550) may manage operation of the video encoder (503).During coding, the controller (550) may assign to each coded picture acertain coded picture type, which may affect the coding techniques thatmay be applied to the respective picture. For example, pictures oftenmay be assigned as one of the following picture types:

An Intra Picture (I picture) may be one that may be coded and decodedwithout using any other picture in the sequence as a source ofprediction. Some video codecs allow for different types of intrapictures, including, for example Independent Decoder Refresh (“IDR”)Pictures. A person skilled in the art is aware of those variants of Ipictures and their respective applications and features.

A predictive picture (P picture) may be one that may be coded anddecoded using intra prediction or inter prediction using at most onemotion vector and reference index to predict the sample values of eachblock.

A bi-directionally predictive picture (B Picture) may be one that may becoded and decoded using intra prediction or inter prediction using atmost two motion vectors and reference indices to predict the samplevalues of each block. Similarly, multiple-predictive pictures can usemore than two reference pictures and associated metadata for thereconstruction of a single block.

Source pictures commonly may be subdivided spatially into a plurality ofsample blocks (for example, blocks of 4×4, 8×8, 4×8, or 16×16 sampleseach) and coded on a block-by-block basis. Blocks may be codedpredictively with reference to other (already coded) blocks asdetermined by the coding assignment applied to the blocks' respectivepictures. For example, blocks of I pictures may be codednon-predictively or they may be coded predictively with reference toalready coded blocks of the same picture (spatial prediction or intraprediction). Pixel blocks of P pictures may be coded predictively, viaspatial prediction or via temporal prediction with reference to onepreviously coded reference picture. Blocks of B pictures may be codedpredictively, via spatial prediction or via temporal prediction withreference to one or two previously coded reference pictures.

The video encoder (503) may perform coding operations according to apredetermined video coding technology or standard, such as ITU-T Rec.H.265. In its operation, the video encoder (503) may perform variouscompression operations, including predictive coding operations thatexploit temporal and spatial redundancies in the input video sequence.The coded video data, therefore, may conform to a syntax specified bythe video coding technology or standard being used.

In an embodiment, the transmitter (540) may transmit additional datawith the encoded video. The source coder (530) may include such data aspart of the coded video sequence. Additional data may comprisetemporal/spatial/SNR enhancement layers, other forms of redundant datasuch as redundant pictures and slices, SEI messages, VUI parameter setfragments, and so on.

A video may be captured as a plurality of source pictures (videopictures) in a temporal sequence. Intra-picture prediction (oftenabbreviated to intra prediction) makes use of spatial correlation in agiven picture, and inter-picture prediction makes uses of the (temporalor other) correlation between the pictures. In an example, a specificpicture under encoding/decoding, which is referred to as a currentpicture, is partitioned into blocks. When a block in the current pictureis similar to a reference block in a previously coded and still bufferedreference picture in the video, the block in the current picture can becoded by a vector that is referred to as a motion vector. The motionvector points to the reference block in the reference picture, and canhave a third dimension identifying the reference picture, in casemultiple reference pictures are in use.

In some embodiments, a bi-prediction technique can be used in theinter-picture prediction. According to the bi-prediction technique, tworeference pictures, such as a first reference picture and a secondreference picture that are both prior in decoding order to the currentpicture in the video (but may be in the past and future, respectively,in display order) are used. A block in the current picture can be codedby a first motion vector that points to a first reference block in thefirst reference picture, and a second motion vector that points to asecond reference block in the second reference picture. The block can bepredicted by a combination of the first reference block and the secondreference block.

Further, a merge mode technique can be used in the inter-pictureprediction to improve coding efficiency.

According to some embodiments of the disclosure, predictions, such asinter-picture predictions and intra-picture predictions are performed inthe unit of blocks. For example, according to the HEVC standard, apicture in a sequence of video pictures is partitioned into coding treeunits (CTU) for compression, the CTUs in a picture have the same size,such as 64×64 pixels, 32×32 pixels, or 16×16 pixels. In general, a CTUincludes three coding tree blocks (CTBs), which are one luma CTB and twochroma CTBs. Each CTU can be recursively quadtree split into one ormultiple coding units (CUs). For example, a CTU of 64×64 pixels can besplit into one CU of 64×64 pixels, or 4 CUs of 32×32 pixels, or 16 CUsof 16×16 pixels. In an example, each CU is analyzed to determine aprediction type for the CU, such as an inter prediction type or an intraprediction type. The CU is split into one or more prediction units (PUs)depending on the temporal and/or spatial predictability. Generally, eachPU includes a luma prediction block (PB), and two chroma PBs. In anembodiment, a prediction operation in coding (encoding/decoding) isperformed in the unit of a prediction block. Using a luma predictionblock as an example of a prediction block, the prediction block includesa matrix of values (e.g., luma values) for pixels, such as 8×8 pixels,16×16 pixels, 8×16 pixels, 16×8 pixels, and the like.

FIG. 6 shows a diagram of a video encoder (603) according to anotherembodiment of the disclosure. The video encoder (603) is configured toreceive a processing block (e.g., a prediction block) of sample valueswithin a current video picture in a sequence of video pictures, andencode the processing block into a coded picture that is part of a codedvideo sequence. In an example, the video encoder (603) is used in theplace of the video encoder (303) in the FIG. 3 example.

In an HEVC example, the video encoder (603) receives a matrix of samplevalues for a processing block, such as a prediction block of 8×8samples, and the like. The video encoder (603) determines whether theprocessing block is best coded using intra mode, inter mode, orbi-prediction mode using, for example, rate-distortion optimization.When the processing block is to be coded in intra mode, the videoencoder (603) may use an intra prediction technique to encode theprocessing block into the coded picture; and when the processing blockis to be coded in inter mode or bi-prediction mode, the video encoder(603) may use an inter prediction or bi-prediction technique,respectively, to encode the processing block into the coded picture. Incertain video coding technologies, merge mode can be an inter pictureprediction submode where the motion vector is derived from one or moremotion vector predictors without the benefit of a coded motion vectorcomponent outside the predictors. In certain other video codingtechnologies, a motion vector component applicable to the subject blockmay be present. In an example, the video encoder (603) includes othercomponents, such as a mode decision module (not shown) to determine themode of the processing blocks.

In the FIG. 6 example, the video encoder (603) includes the interencoder (630), an intra encoder (622), a residue calculator (623), aswitch (626), a residue encoder (624), a general controller (621), andan entropy encoder (625) coupled together as shown in FIG. 6 .

The inter encoder (630) is configured to receive the samples of thecurrent block (e.g., a processing block), compare the block to one ormore reference blocks in reference pictures (e.g., blocks in previouspictures and later pictures), generate inter prediction information(e.g., description of redundant information according to inter encodingtechnique, motion vectors, merge mode information), and calculate interprediction results (e.g., predicted block) based on the inter predictioninformation using any suitable technique. In some examples, thereference pictures are decoded reference pictures that are decoded basedon the encoded video information.

The intra encoder (622) is configured to receive the samples of thecurrent block (e.g., a processing block), in some cases compare theblock to blocks already coded in the same picture, generate quantizedcoefficients after transform, and in some cases also intra predictioninformation (e.g., an intra prediction direction information accordingto one or more intra encoding techniques). In an example, the intraencoder (622) also calculates intra prediction results (e.g., predictedblock) based on the intra prediction information and reference blocks inthe same picture.

The general controller (621) is configured to determine general controldata and control other components of the video encoder (603) based onthe general control data. In an example, the general controller (621)determines the mode of the block, and provides a control signal to theswitch (626) based on the mode. For example, when the mode is the intramode, the general controller (621) controls the switch (626) to selectthe intra mode result for use by the residue calculator (623), andcontrols the entropy encoder (625) to select the intra predictioninformation and include the intra prediction information in thebitstream; and when the mode is the inter mode, the general controller(621) controls the switch (626) to select the inter prediction resultfor use by the residue calculator (623), and controls the entropyencoder (625) to select the inter prediction information and include theinter prediction information in the bitstream.

The residue calculator (623) is configured to calculate a difference(residue data) between the received block and prediction resultsselected from the intra encoder (622) or the inter encoder (630). Theresidue encoder (624) is configured to operate based on the residue datato encode the residue data to generate the transform coefficients. In anexample, the residue encoder (624) is configured to convert the residuedata from a spatial domain to a frequency domain, and generate thetransform coefficients. The transform coefficients are then subject toquantization processing to obtain quantized transform coefficients. Invarious embodiments, the video encoder (603) also includes a residuedecoder (628). The residue decoder (628) is configured to performinverse-transform, and generate the decoded residue data. The decodedresidue data can be suitably used by the intra encoder (622) and theinter encoder (630). For example, the inter encoder (630) can generatedecoded blocks based on the decoded residue data and inter predictioninformation, and the intra encoder (622) can generate decoded blocksbased on the decoded residue data and the intra prediction information.The decoded blocks are suitably processed to generate decoded picturesand the decoded pictures can be buffered in a memory circuit (not shown)and used as reference pictures in some examples.

The entropy encoder (625) is configured to format the bitstream toinclude the encoded block. The entropy encoder (625) is configured toinclude various information according to a suitable standard, such asthe HEVC standard. In an example, the entropy encoder (625) isconfigured to include the general control data, the selected predictioninformation (e.g., intra prediction information or inter predictioninformation), the residue information, and other suitable information inthe bitstream. Note that, according to the disclosed subject matter,when coding a block in the merge submode of either inter mode orbi-prediction mode, there is no residue information.

FIG. 7 shows a diagram of a video decoder (710) according to anotherembodiment of the disclosure. The video decoder (710) is configured toreceive coded pictures that are part of a coded video sequence, anddecode the coded pictures to generate reconstructed pictures. In anexample, the video decoder (710) is used in the place of the videodecoder (310) in the FIG. 3 example.

In the FIG. 7 example, the video decoder (710) includes an entropydecoder (771), an inter decoder (780), a residue decoder (773), areconstruction module (774), and an intra decoder (772) coupled togetheras shown in FIG. 7 .

The entropy decoder (771) can be configured to reconstruct, from thecoded picture, certain symbols that represent the syntax elements ofwhich the coded picture is made up. Such symbols can include, forexample, the mode in which a block is coded (such as, for example, intramode, inter mode, bi-predicted mode, the latter two in merge submode oranother submode), prediction information (such as, for example, intraprediction information or inter prediction information) that canidentify certain sample or metadata that is used for prediction by theintra decoder (772) or the inter decoder (780), respectively, residualinformation in the form of, for example, quantized transformcoefficients, and the like. In an example, when the prediction mode isinter or bi-predicted mode, the inter prediction information is providedto the inter decoder (780); and when the prediction type is the intraprediction type, the intra prediction information is provided to theintra decoder (772). The residual information can be subject to inversequantization and is provided to the residue decoder (773).

The inter decoder (780) is configured to receive the inter predictioninformation, and generate inter prediction results based on the interprediction information.

The intra decoder (772) is configured to receive the intra predictioninformation, and generate prediction results based on the intraprediction information.

The residue decoder (773) is configured to perform inverse quantizationto extract de-quantized transform coefficients, and process thede-quantized transform coefficients to convert the residual from thefrequency domain to the spatial domain. The residue decoder (773) mayalso require certain control information (to include the QuantizerParameter (QP)), and that information may be provided by the entropydecoder (771) (data path not depicted as this may be low volume controlinformation only).

The reconstruction module (774) is configured to combine, in the spatialdomain, the residual as output by the residue decoder (773) and theprediction results (as output by the inter or intra prediction modulesas the case may be) to form a reconstructed block, that may be part ofthe reconstructed picture, which in turn may be part of thereconstructed video. It is noted that other suitable operations, such asa deblocking operation and the like, can be performed to improve thevisual quality.

It is noted that the video encoders (303), (503), and (603), and thevideo decoders (310), (410), and (710) can be implemented using anysuitable technique. In an embodiment, the video encoders (303), (503),and (603), and the video decoders (310), (410), and (710) can beimplemented using one or more integrated circuits. In anotherembodiment, the video encoders (303), (503), and (503), and the videodecoders (310), (410), and (710) can be implemented using one or moreprocessors that execute software instructions.

Aspects of the disclosure provide techniques for affine model predictionin video coding (encoding/decoding). Generally, a motion vector for ablock can be coded either in an explicit way, to signal the differenceto a motion vector predictor (e.g., advanced motion vector prediction orAMVP mode); or in an implicit way, to be indicated completely from onepreviously coded or generated motion vector. The later one is referredto as merge mode, meaning the current block is merged into a previouslycoded block by using its motion information.

Both the AMVP mode and the merge mode construct candidate list duringdecoding. FIG. 8 shows an example of spatial and temporal candidates insome examples.

For the merge mode in the inter prediction, merge candidates in acandidate list are primarily formed by checking motion information fromeither spatial or temporal neighboring blocks of the current block. Inthe FIG. 8 example, candidate blocks A1, B1, B0, A0 and B2 aresequentially checked. When any of the candidate blocks are validcandidates, for example, are coded with motion vectors, then, the motioninformation of the valid candidate blocks can be added into the mergecandidate list. Some pruning operation is performed to make sureduplicated candidates will not be put into the list again. The candidateblocks A1, B1, B0, A0 and B2 are adjacent to corners of the currentblock, and are referred to as corner candidates.

After spatial candidates, temporal candidates are also checked into thelist. In some examples, the current block's co-located block in aspecified reference picture is found. The motion information at C0position (bottom right corner of the current block) of the co-locatedblock will be used as temporal merge candidate. If the block at thisposition is not coded in inter mode or not available, C1 position (atthe outer bottom right corner of the center of the co-located block)will be used instead. The present disclosure provides techniques tofurther improve merge mode.

The advanced motion vector prediction (AMVP) mode in HEVC refers tousing spatial and temporal neighboring blocks' motion information topredict the motion information of the current block, while theprediction residue is further coded. Examples of spatial and temporalneighboring candidates are shown in FIG. 8 as well.

In some embodiments, in AMVP mode, a two-candidate motion vectorpredictor list is formed. For example, the list includes a firstcandidate predictor and a second candidate predictor. The firstcandidate predictor is from the first available motion vector from theleft edge, in the order of spatial A0, A1 positions. The secondcandidate predictor is from the first available motion vector from thetop edge, in the order of spatial B0, B1 and B2 positions. If no validmotion vector can be found from the checked locations for either theleft edge or the top edge, no candidate will be filled in the list. Ifthe two candidates available and are the same, only one will be kept inthe list. If the list is not full (with two different candidates), thetemporal co- located motion vector (after scaling) from C0 location willbe used as another candidate. If motion information at C0 location isnot available, location C1 will be used instead.

In some examples, if there are still no enough motion vector predictorcandidates, zero motion vector will be used to fill up the list.

In some embodiments, prediction offsets can be signaled on top ofexisting merge candidates. For example, a technique that is referred toas ultimate motion vector expression (UMVE) uses a special merge mode inwhich an offset (both magnitude and direction) on top of the existingmerge candidates is signaled. In this technique, a few syntax elements,such as a prediction direction IDX, a base candidate IDX, a distanceIDX, a search direction IDX, and the like, are signaled to describe suchan offset. For example, the prediction direction IDX is used to indicatewhich of the prediction directions (temporal prediction direction, e.g.,L0 reference direction, L1 reference direction or L0 and L1 referencedirections) is used for UMVE mode. The base candidate IDX is used toindicate which of the existing merge candidates is used as the startpoint (based candidate) to apply the offset. The distance IDX is used toindicate how large the offset is from the starting point (along x or ydirection, but not both). The offset magnitude is chosen from a fixnumber of selections. The search direction IDX is used to indicate thedirection (x or y, + or − direction) to apply the offset.

In an example, assuming the starting point MV is MV_S, the offset isMV_offset. Then the final MV predictor will be MV_final=MV_S+MV_offset.

FIG. 9 shows examples for UMVE according to an embodiment of thedisclosure. In an example, the starting point MV is shown by (911) (forexample according to the prediction direction IDX and base candidateIDX), the offset is shown by (912) (for example according to thedistance IDX and the search direction IDX), and the final MV predictoris shown by (913) in FIG. 9 . In another example, the starting point MVis shown by (921) (for example according to the prediction direction IDXand base candidate IDX), the offset is shown by (922) (for exampleaccording to the distance IDX and the search direction IDX), and thefinal MV predictor is shown by 923 in FIG. 9 .

FIG. 10 shows examples for UMVE according to an embodiment of thedisclosure. For example, the starting point MV is shown by (1011) (forexample according to the prediction direction IDX and base candidateIDX). In the FIG. 10 example, 4 search directions, such as +Y, −Y, +Xand −X, are used, and the four search directions can be indexed by 0, 1,2, 3. The distance can be indexed by 0 (0 distance to the starting pointMV), 1 (1s to the starting point MV), 2 (2s to the starting point MV), 3(3s to the starting point), and the like. Thus, when the searchdirection IDX is 3, and the distance IDX is 2, the final MV predictor isshown as 1015.

In another example, the search direction and the distance can becombined for indexing. For example, the starting point MV is shown by(1021) (for example according to the prediction direction IDX and basecandidate IDX). The search direction and the distance are combined to beindexed by 0-12 as shown in FIG. 10 .

According to an aspect of the disclosure, affine motion compensation, bydescribing a 6-parameter (or a simplified 4-parameter) affine model fora coding block, can efficiently predict the motion information forsamples within the current block. More specifically, in an affine codedor described coding block, different part of the samples can havedifferent motion vectors. The basic unit to have a motion vector in anaffine coded or described block is referred to as a sub-block. The sizeof a sub-block can be as small as 1 sample only; and can be as large asthe size of current block.

When an affine mode is determined, for each sample in the current block,its motion vector (relative to the targeted reference picture) can bederived using such a model (e.g., 6 parameter affine motion model or 4parameter affine motion model). In order to reduce implementationcomplexity, affine motion compensation is performed on a sub-blockbasis, instead of on a sample basis. That means, each sub-block willderive its motion vector and for samples in each sub-block, the motionvector is the same. A specific location of each sub-block is assumed,such as the top-left or the center point of the sub-block, to be therepresentative location. In one example, such a sub-block size contains4×4 samples.

In general, an affine motion model has 6 parameters to describe themotion information of a block. After the affine transformation, arectangular block will become a parallelogram. In an example, the 6parameters of an affine coded block can be represented by 3 motionvectors at three different locations of the block.

FIG. 11 shows an example of a block (1100) with an affine motion model.The block (1100) uses motion vectors {right arrow over (v₀)},{rightarrow over (v₁)}, and {right arrow over (v₂)}at three corner locationsA, B and C to describe the motion information of the affine motion modelused for the block (1100). These locations A, B and C are referred to ascontrol points.

In simplified example, an affine motion model uses 4 parameters todescribe the motion information of a block based on an assumption thatafter the affine transformation, the shape of the block does not change.Therefore, a rectangular block will remain a rectangular and same aspectratio (e.g., height/width) after the transformation. The affine motionmodel of such a block can be represented by two motion vectors at twodifferent locations, such as at corner locations A and B.

FIG. 12 shows examples of affine transformation for a 6-parameter affinemode (using 6-parameter affine model) and a 4-parameter affine mode(using 4-parameter affine model).

In an example, when assumptions are made such that the object only haszooming and translational motions, or the object only has rotation andtranslation models, then the affine motion model can be furthersimplified to a 3-parameter affine motion model with 2 parameters toindicate the translational part and 1 parameter to indicate either ascaling factor for zooming or an angular factor for rotation.

According to an aspect of the disclosure, when affine motioncompensation is used, two signaling techniques can be used. The twosignaling techniques are referred to as a merge mode based signalingtechnique and a residue (AMVP) mode based signaling technique.

For the merge mode based signaling technique, the affine information ofthe current block is predicted from previously affine coded blocks. Inone method, the current block is assumed to be in the same affine objectas the reference block, so that the MVs at the control points of thecurrent block can be derived from the reference block's model. The MVsat the current block' other locations are just linearly modified in thesame way as from one control point to another in the reference block.This method is referred to as model based affine prediction. In anothermethod, neighboring blocks' motion vectors are used directly as themotion vectors at current block's control points. Then motion vectors atthe rest of the block are generated using the information from thecontrol points. This method is referred as control point based affineprediction. In either method, no residue components of the MVs atcurrent block are to be signaled. In other words, the residue componentsof the MVs are assumed to be zero.

For the residue (AMVP) mode based signaling technique, affineparameters, or the MVs at the control points of the current block, areto be predicted. Because there are more than one motion vectors to bepredicted, the candidate list for motion vectors at all control pointsis organized in grouped way such that each candidate in the listincludes a set of motion vector predictors for all control points. Forexample, candidate 1={predictor for control point A, predictor forcontrol point B, predictor for control point C}; candidate 2={predictorfor control point A, predictor for control point B, predictor forcontrol point C}, etc. The predictor for the same control point indifferent candidates can be the same or different. The motion vectorpredictor flag ((mvp_10_flag for List 0 or mvp_11_flag for List 1) willbe used to indicate which candidate from the list is chosen. Afterprediction, the residue part of the parameter, or the differences of theactual MVs to the MV predictors at the control points, are to besignaled. The MV predictor at each control point can also come frommodel based affine prediction from one of its neighbors, using themethod described from the above description for merge mode based osignaling technique.

In some related methods, affine parameters for a block can be eitherpurely derived from neighboring block's affine model or control points'MV predictor, or from explicitly signal the MV differences at thecontrol points. However, in many cases the non-translational part of theaffine parameters is very close to zero. Using unrestricted MVdifference coding to signal the affine parameters has redundancy.

Aspects of the disclosure provide new techniques to improve theefficiency of affine motion compensation. More specifically, to predictaffine model parameters in a more efficient way. In the disclosure,affine motion information of a block is represented by using an affinemodel parameter prediction. A prediction candidate (or predictor) thatis used can be similar to the affine merge candidates or affine AMVPcandidates as described above. A delta of motion information between apredicted block and a prediction candidate block can be represented intwo ways: 1) using deltas of affine parameters; 2) using deltas ofmotion vectors of control points of a current block. The delta of eachof the affine parameters or the delta of each of the motion vectors ofthe control points can be a respective set of predetermined offsetvalues. The predetermined offset values can be considered as somerefinements or offsets around the corresponding base parameters of thepredictor. The encoder evaluates a best option from the predeterminedoffset values and signals an index of the offset choice to the decoder.The decoder restores the affine model parameters or motion vectors ofcontrol points according to the signaled index.

In the disclosure, an affine merge candidate can include two or morecontrol points. Each of the control points can include one or more MVs.Offsets (e.g., distance and direction) for each of the control point'sMVs can be sent by the encoder to represent an affine motion. A distanceoffset table size can be variable and can be signaled or predefined. Avalue range of the distance offset can be variable and determined via ahigh-level syntax. A set of candidate step sizes are provided. An indexof the selection can be signaled, such as at slice level. The encodercan use data from previous coded picture to make decision.

In some embodiments, the current block has N control points (CPs), whereN is a positive integer and more than one. For each of the N CPs, azero_MVD flag is used to indicate whether a motion vector difference(MVD) is zero. The MVD is a difference between a MV of the control pointand a motion vector prediction (MPV) of the control point. If first(N-1) CPs have zero_MVD flag that equals to one (i.e., zero MVD), a lastCP's zero_MVD flag is inferred to be zero (i.e., none zero MVD).

In some embodiments, the disclosure includes affine parameter basedaffine mode offset signaling.

The method can be illustrated based on a 4-parameter affine model with 2control points (e.g., CP0 and CP1), as shown in FIG. 13 . However, FIG.13 is a merely example and the methods in the disclosure can be extendedto other motion models, or affine models with different numbers ofparameters. In some embodiments, the model used may not always be affinemodel, but possibly other types of motion.

In an example, a 4-parameter affine model is described, such as shown byEq. 1,

$\begin{matrix}\{ {\begin{matrix}{x^{\prime} = {{\rho\cos{\theta \cdot x}} + {\rho\sin{\theta \cdot y}} + c}} \\{y^{\prime} = {{{- \rho}\sin{\theta \cdot x}} + {\rho\cos{\theta \cdot y}} + f}}\end{matrix},}  & {{Eq}.1}\end{matrix}$

where p is the scaling factor for zooming, θ is the angular factor forrotation, and (c, f) is the motion vector to describe the translationalmotion. (x, y) is a pixel location in the current picture, (x′, y′) is acorresponding pixel location in the reference picture.

Let a =ρcos θ, and let b =ρsin θ, Eq. 1 may become the following form asin Eq. 2

$\begin{matrix}\{ \begin{matrix}{x^{\prime} = {{a \cdot x} + {b \cdot y} + c}} \\{y^{\prime} = {{{- b} \cdot x} + {a \cdot y} + f}}\end{matrix}  & {{Eq}.2}\end{matrix}$

Thus, a 4-parameter affine model can be represented by a set ofmodel-based parameters {ρ, θ, c, f}, or {a, b, c, f}. Based on Eq. 2,motion vector (MV_(x), MV_(y)) at a pixel position (x, y) can bedescribed as in Eq. 3.

$\begin{matrix}\{ {\begin{matrix}{{MV}_{x} = {{x^{\prime} - x} = {{ax} + {by} + c}}} \\{{MV}_{y} = {{y^{\prime} - y} = {{- {bx}} + {ay} + f}}}\end{matrix},}  & {{Eq}.3}\end{matrix}$

where V_(x), is a horizontal motion vector value, and V_(y) is avertical motion vector value.

The 4-parameter affine model can also be represented by the motionvectors of two control points, CP0 and CP1, of the block. Similarly,three control points may be required to represent a 6-parameter affinemodel. To derive the motion vector at position (x, y) in the currentblock, a following Eq. 4 can be used:

$\begin{matrix}\{ \begin{matrix}{v_{x} = {{\frac{( {v_{1x} - v_{0x}} )}{w}x} - {\frac{( {v_{1y} - v_{0y}} )}{w}y} + v_{0x}}} \\{v_{y} = {{\frac{( {v_{1y} - v_{0y}} )}{w}x} + {\frac{( {v_{1x} - v_{0x}} )}{w}y} + v_{0y}}}\end{matrix}  & {{Eq}.4}\end{matrix}$

where (v_(0x), v_(0y)) is a motion vector of the top-left corner controlpoint, CP0 as depicted in FIG. 13 , and (v_(1x), v_(1y)) is a motionvector of the top-right corner control point, CP1 as depicted in FIG. 13. Accordingly, in the control-point based model, the affine model of theblock can be represented by {v_(0x), v_(0y), v_(1x), v_(1y), }.

The affine model can be predicted by using MVs of the control pointslocated at two or three corners of the current block, either by themodel-based prediction or corner control-points based prediction. Afterthe motion vector prediction for the two or three control points, the MVdifferences (MVDs) of the control points can be signaled. A set ofpre-defined delta values can be applied to represent a real MVdifference.

FIG. 14 shows a first flow chart that outlines an exemplary process(1400) of control-point motion vector based affine merge with offset. Asshown in FIG. 14 , the process (1400) can start from (S1410) where amerge flag and an affine_merge_ with offset usage flag of the currentblock are signaled by the encoder and coded by the decoder subsequently.When both the merge flag and the affine_ merge_ with offset usage flagare false, the process (1400) proceeds to (S1420) where a traditionalmerge mode is applied to predict the current block. When both the mergeflag and the affine_merge_ with offset usage flag are true, the process(1400) proceeds to (S1430). At (S1430), when more than one predictorcandidates are used to decide a base predictor, a base predictor indexis signaled by the encoder at (S1450). Otherwise, the process (1400)proceeds to (S1440) where a predefined base predictor index is applied.

The process (1400) then proceeds to (S1460) when the base predictor isdefined either at (S1430) or (S1440). At (S1460), for each of thecontrol points of the current block, a Zero_MVD flag is signaled by theencoder. It should be noted that when all other CPs have a Zero_ MVDthat is equal to one (i.e., true), a last CP is inferred to be zero(i.e., false) without signaling.

Still referring to (S1460), when the Zero_MVD flag is true, the process(1400) proceeds to (S1480) where a MV of a control point of the basepredictor is set as a MV of a control point of the block. When theZero_MVD flag is false, the process (1400) proceeds to (S1470) where aDistance Offset Index and an Offset Direction Index for the CP of theblock is signaled by the encoder. At (S1480), the decoder subsequentlydecodes the distance offset index to determine a distance offset value,and decode the offset direction index to determine an offset direction.The distance offset value and the offset direction are accordinglyapplied onto the motion vector of the control point of the basepredictor to generate the motion vector of the control point of theblock in the current picture.

In an embodiment, a usage flag is signaled after a merge flag toindicate whether the proposed method is used or not. Since the proposedmethod is applied for affine inter prediction, when the usage flag issignaled to be true, an affine flag for the current block can beinferred to be true. Accordingly, the signaling of affine flag can beskipped.

In another embodiment, a usage flag is signaled after the merge flag andthe affine flag when both the merge flag and the affine flag are true.Otherwise, when the merge flag is false or affine flag is false, theusage flag is inferred to be false.

When the proposed method is used, a base index can be signaled toindicate which affine predictor candidate is used as a base predictor.In an embodiment, when only one affine predictor candidate is allowed,the base index can be skipped, and a predefined affine predictorcandidate can be used as the base predictor. In an embodiment, the baseindex indicates which candidate from affine merge candidate list to beused as the base predictor. In another embodiment, the base indexindicates which affine MVP candidate to be used as the base predictor.When an affine merge candidate or an affine MVP candidate isunavailable, the proposed method may be disabled or may be enabled witha default base affine model, such as translational model, or zero motionvectors, etc.

For each of the control points of the current block, a Zero_MVD flag canbe used to indicate whether the motion vector difference (MVD) is zerofor the control point. When the MVD is indicated to be zero, the MV ofthe control point is set as the MV of the corresponding control point ofthe base predictor.

In an embodiment, Zero_MVD flag may be explicitly signaled for allcontrol points. In another embodiment, when all previous control pointshave a Zero_MVD flag that is signaled to be true, the last controlpoint's Zero_MVD flag can be inferred to be false.

When the MVD value is not zero for a control point, a distance offsetindex and an offset direction index can be signaled to represent the MVDvalue of the corresponding control point. Table 1 is an example of thesignals applied for a 4-parameter affine model, which has two controlpoints.

TABLE 1 Signals applied for a 4-parameter affine model Usage Base CP0CP0 CP0 CP1 CP1 CP1 Flag Predictor Zero_MVD Distance Direction Zero_MVDDistance Direction Index Flag Index Index Flag Index Index

For a 6-parameter affine model, where 3 control points (CP) can besignaled, the signaling can shows as Table 2.

TABLE 2 Signals applied for a 6-parameter affine model Usage Base CP0CP0 CP0 CP1 CP1 CP1 CP2 CP2 CP2 Flag Predictor zero_MVD DistanceDirection zero_MVD Distance Direction zero_MVD Distance Direction IndexFlag Index Index Flag Index Index Flag Index Index

In the proposed method, a pixel distance offset can be signaled by adistance offset index. In a distance offset table, a distance offsetindex is mapped to a corresponding distance offset in number of pixels.The distance offset value can be an integer or fractional values. Thedistance offset value can be further applied to the base predictor'smotion vector value.

In an embodiment, a distance offset table with a size of four indicescan be shown in Table 3. The distance offset values in Table 3 can be{½, 1, 2, 4}, in terms of pixels.

TABLE 3 A distance offset table with a size of four indices Distance IDX0 1 2 3 Distance offset ½-pel 1-pel 2-pel 4-pel

In another embodiment, Table 4 illustrates that the distance offsetvalues can be {⅛, ¼, ½,1}, in terms of pixels.

TABLE 4 Another distance offset table with a size of four indicesDistance IDX 0 1 2 3 Distance offset ⅛-pel ¼-pel ½-pel 1-pel

In an embodiment, a distance offset table with size of five indices canbe shown in Table 5. The distance offset values in Table 5 can be {½, 1,2, 4, 8}, in terms of pixels.

TABLE 5 A distance offset table with a size of five indices Distance IDX0 1 2 3 4 Distance offset ½-pel 1-pel 2-pel 4-pel 8-pel

In another embodiment, a mapping table of distance offset values witheight indices can be shown in Table 6. The distance offset values can bein a range from ¼pixels to 32 pixels.

TABLE 6 A distance offset table with a size of eight indices DistanceIDX 0 1 2 3 4 5 6 7 Distance offset ¼-pel ½-pel 1-pel 2-pel 4-pel 8-pel16-pel 32-pel

In another embodiment, a mapping table of distance offset values witheight indices can be shown in Table 7. The distance offset values inTable 7 can be in a range from 1/16 pixels to 8 pixels.

TABLE 7 Another distance offset table with a size of eight indicesDistance IDX 0 1 2 3 4 5 6 7 Distance offset 1/16-pel ⅛-pel ¼-pel ½-pel1-pel 2-pel 4-pel 8-pel

It should be noted that the tables mentioned above are merely examples.The distance index in a distance offset table can have various sizes,such as four, five or eight that are illustrated in Table 3, 5 and 6respectively. In the distance offset table, each of the distance offsetindices is mapped to a respective distance offset. The distance offsetcan also have different values or be in different ranges.

In an embodiment, the size of the distance offset table can be same forall control points of the current block.

In another embodiment, the size of the distance offset table can bedifferent for each of the control points. For example, for a 4-parameteraffine model with two control points, the first control point CP0 canhave a distance offset table with five entries (indices). The secondcontrol point CP1 can have a distance offset table with four entries(indices). The possible table size may not be limited to examplesmentioned above in Tables 3-7.

In the proposed method, an offset direction index is mapped to one ormore offset directions, such as an x-axis and a y-axis. The offsetdirection index corresponds to directions of the MVD relative to thebase predictor's MV value. Each of the offset directions is correlatedto a component of the distance offset that can be applied to the MV ofthe based predictor.

In an embodiment, the offset direction index can include four directionsas shown in Table 8. Each of the four directions can include arespective x-axis (x-offset direction component) and a respective y-axis(y-offset direction component). As shown in Table 8, the MVD exists oneither the x-axis or the y-axis, but not on both axes.

TABLE 8 Mapping of direction IDXs to directions Offset Direction IDX 0001 10 11 x-axis + − 0 0 y-axis 0 0 + −

In another example, the MVD can exist on x-axis only, y-axis only, orboth axes, which can be illustrated in Table 9. As shown in Table 9, theoffset direction index can include eight directions, and one of theeight directions can be applied.

TABLE 9 Mapping of direction IDXs to directions Offset Direction IDX 000001 010 011 100 101 110 111 x-axis + + 0 − − − 0 + y-axis 0 + − − 0 + +−

In the disclosure, when the proposed method is applied, similar toaffine merge mode, an inter prediction direction of the current blockcan use an inter prediction direction from the base predictor.

In some embodiments, when the base predictor's inter prediction isuni-directional, which means the motion vectors of the control points ofthe current block are pointing to only one reference picture alongeither forward or backward direction, the motion vector difference (MVD)between the MV of the current block and the MVP of the current block(i.e., the MV of the base predictor) can be derived based on a validinter prediction direction. For each of the offset directions mentionedabove, the distance offset value can be applied to the motion vectorvalue of the base predictor's corresponding control point along anoffset direction component (e.g., x-axis, y-axis) that is not zero.

For example, the current block can have two control points CP0 and CP1.The control point CP0 can have a motion vector MV0 (v_(0x), v_(0y)) andthe control point CP1 can have a MV1 (v_(1z), v_(1y)). The basepredictor of the current block can have two control points CP0_(p) andCP1_(p). The control point CP0_(p) can have a motion vector MVPO(v_(0px), v_(0py)) and the control point CP1_(p) can have a motionvector MVP1 (v_(1px), v_(1py)). When the CP0's MVD flag indicates thatthe MVD of CP0 is a non-zero MVD, a distance offset of the CP0 issignaled to be 1-pel, an offset direction of the CP0 is signaled to be“+” on x-axis and 0 on y-axis, the CP1's MVD flag indicates the MVD ofCP1 is non-zero, a distance offset of the CP1 is signaled to be 2-pel,and an offset direction of the CP1 is 0 on x-axis and “-” on y-axis,motion vector values for CP0 and CP1 can be derived as follows:

MV0(v_(0x), v_(0y))=MVP0 (v_(0px), v_(0py))+MV(1, 0), so thatv_(0x)=v_(0px)+1, v_(0y)=v_(0py);

MV1(v_(1x), v_(1y))=MVP1 (v_(1px), v_(1py))+MV(0, −2), so thatv_(1x)=v_(1px), v_(1py)−2.

where MV(1,0) is the MVD of the CP0 of the current block and MV (0,−2)is the MVD of the CP1 of the current block.

In some embodiments, when the base predictor's inter prediction isbi-directional, each of the control points of the current block can havemotion vectors on both inter prediction directions, and differentmethods can be applied to derive the motion vectors on the two validdirections.

In an embodiment, the signaled distance offset and offset direction canbe applied to MVPs of the control point on both inter predictiondirections in a same way. For example, a distance offset index and anoffset direction index are signaled for a control point of the currentblock which has two motion vector predictor values on two respectiveprediction directions, and the distance offset index and the offsetdirection index can be applied to the two motion vector predictor valuesof the control point in the same way. The signaled distance offset andoffset direction can be applied on top of the MVP value on an interdirection with reference list LO. The same distance offset and offsetdirection can be applied on top of the MVP value on the inter directionwith reference list L1.

For example, the current block can have two control points CP0 and CP1.The CP0 has a motion vectors L0_MV0 (L0_v_(0x), L0_v_(0y)) on an interdirection L0, and a motion vector L1_ MV0(L1_v_(0x), L1_v_(0y)) on aninter direction L1. The CP1 has a motion vectors L0_MV1 (L0_v_(1x),L0_v_(1y)) on the inter direction L0, and a motion vector L1_MV1(L0_v_(1x), L0_v_(1y)) on the inter direction L1. The base predictor canhave two control points CP0_(p) and CP1_(p). The CP0_(p) can have amotion vectors L0_MVP0 (L0_v_(0px), L0_v_(0py)) on the inter directionL0, and a motion vector L1_MVP0 (L1_v_(0px), L1_v_(0py)) on the interdirection L1. The CP1 _(p) can have a motion vector L0_MVP1 (L0_v_(1px),L0_v_(1py)) on the inter direction L0, and a motion vector L1_MVP1(L1_v_(1px), L1_v_(1px)) on the inter direction L1. When the CP0's MVDflag indicates that the MVD of the CP0 is a non-zero MVD, a distanceoffset of CP0 is signaled to be 1-pel, an offset direction of CP0 issignaled to be “+” on x-axis and 0 on y-axis, the CPI's MVD flagindicates that the MVD of the CP1 is a non-zero MVD, a distance offsetof CP1 is signaled to be 2-pel, an offset direction of CP1 is 0 onx-axis and “−” on y-axis, the derived motion vector values for CP0 andCP1 of the current block can be:

L0_MV0(L0_v_(0x), L0_v_(0y))=L0_MVP0(L0_v_(0px), L0_v_(0py))+MV(1, 0),so that L0_v_(0x)=L0_v_(0px)+1, L0_v_(0y)=L0_v_(0py);

L1_MV0(L1_v_(0x), L1_v_(0y))=L1_MVP0(L1_v_(0px), L1_v_(0py))+MV(1, 0),so that L1_v_(0x)=L1_v_(0px)+1, L1_v_(0y)=L1_v_(0py);

L0_MV1(L0_v_(1x), L0_v_(1y))=L0_MVP1(L1_v_(1px),L0_v_(1py))+L0_MV(0,−2), so that L0_v_(1x)=L0_v_(1px),L0_v_(1y)=L0_v_(1py)−2; and

L1_MV1(L1_v_(1x), L1_v_(1y))=L1_MVP1(L1_v_(1px),L1_v_(1py))+L1_MV(0,−2), so that L1_v_(1x)=L1_v_(1px),L1_v_(1y)=L1_v_(1py)—2.

In another embodiment, the signaled distance offset and offset directioncan be applied to MVPs of the control point on two inter predictiondirections with a same distance offset but mirrored offset directions.For example, a distance offset index and an offset direction index aresignaled for a control point of the current block which has two motionvector predictor values on two respective prediction directions, but thedistance offset index and the offset direction index can be applied tothe two motion vector predictor values of the control point in adifferent way. The signaled distance offset and offset direction can beapplied on top of the MVP value on the inter direction with referencelist L0, and the same distance offset and an opposite offset directioncan be applied on top of the MVP value on the inter direction withreference list L1.

For example, the current block can have two control points CP0 and CP1.The CP0 has a motion vectors L0_MV0 (L0_v_(0x), L0_v_(0y)) on an interdirection L0, and a motion vector L1_MV0 (L1_v_(0x), L1_v_(0y)) on aninter direction L1. The CP1 has a motion vectors L0_MV1 (L0_v_(1x),L0_v_(0y)) on the inter direction L0, and a motion vector L1_MV1(L0_v_(1x), L0_v_(1y)) on the inter direction L1. The base predictor canhave two control points CP0_(p) and CP1_(p). The CP0_(p) can have amotion vectors L0_MVP0 (L0_v_(0px), L0_v_(0py)) on the inter directionL0, and a motion vector L1_MVP0 (L1_v_(0px), :1_v_(0py)) on the interdirection L1. The CP1_(p) can have a motion vector L0_MVP1 (L0_v_(1px),L0_v_(1py)) on the inter direction L0, and a motion vector L1_MVP1(L1_v_(1px), L1_v_(1py)) on the inter direction L1. When the CP0's MVDflag indicates that the MVD of the CP0 is a non-zero MVD, a distanceoffset of CP0 is signaled to be 1-pel, an offset direction of CP0 issignaled to be “+” on x-axis and 0 on y-axis, the CP1's MVD flagindicates that the MVD of the CP1 is a non-zero MVD, a distance offsetof CP1 is signaled to be 2-pel, an offset direction of CP1 is 0 onx-axis and “−” on y-axis, the derived motion vector values for CP0 andCP1 of the current block can be:

L0_MV0 (L0_v_(0x), L0_v_(0y))=L0_MVP0 (L0_v_(0px), L0_v_(0py))+MV(1, 0),so that L0_v_(0x)=L0_v_(0px)+1, L0_v_(0y)=L0_v_(0py);

L1_MV0 (L1_v_(0x), L1_v_(0y))=L1_MVP0 (L1_v_(0px), L1_v_(0py))+MV(−1,0), so that L1_v_(0x)=L1_v_(0px)−2, L1_v_(0py);

L0_MV1(L0_v_(1x), L0_v_(1y))=L0_MVP1 (L0_v_(1px),L0_v_(1py))+L0_MV(0,−2), so that L0_v_(1x)=L0_v_(1px),L0_v_(1y)=L0_v_(1py)−2; and

L1_MV1(L1 _v_(1x)L1_v_(1y))=L1_MVP1 (L1_v_(1px), L1_v_(1py)) +L1_MV(0,2), so that L1_v_(1x)=L1_v_(1px), L1_v_(1y)=L1_v_(1py)+2.

In another embodiment, the signaling of distance offset index and offsetdirection is done separately for each of the inter predictiondirections. So that for each of the control points with bi-directionalaffine inter prediction, two distance offset indices and two offsetdirections can be signaled by the encoder.

In an embodiment, a same distance offset mapping table and/or the offsetdirection mapping table can be used for all cases that are mentionedabove.

In another embodiment, a different distance offset mapping table and/ora different offset direction mapping table can be used. A determinedmapping table can be signaled at a sequence level, a slice level, a tilelevel, a tile group level, or a block level.

In another embodiment, a different distance offset mapping table and/ora different offset direction mapping table can be used for each sequencewithout signaling. A determination of the mapping table can be madebased on resolution of the encoded video sequence, profile/level ofcoded, user configuration, etc.

In an embodiment, the above derivation can be applied on each of thecontrol points' MV difference (MVD) for the affine mode.

In another embodiment, after coding of MV difference for the firstcontrol point, the MV difference of the first control point can be usedto predict other MV difference(s) before performing MVD coding for otherMVD(s) of other control points. Such a process is referred as MVDprediction. After the MVD prediction, a MVD prediction error can becoded by using the methods proposed in the disclosure, which applies apre-defined set of values to approximate the actual value of the MVD.

In the proposed method, on the encoder side, different search methodscan be applied to determine best parameters to use for the proposed MVDcoding methods.

In an embodiment, all possible combinations of a base predictor, a MVDcoding flag for each of the control points, a distance offset index, andan offset direction index, can be tested to find a best combination withoptimal rate-distortion cost.

In another embodiment, two rounds of search can be applied. In the firstround of search, a fixed distance offset value can be applied along withall possible combinations of the MVD flag and offset directions. Basedon the best MVD flag and/or offset direction determined by the firstround of search, the second round search can test all remaining distanceoffset indices to find a final best prediction.

In the disclosure, the affine model can also be predicted by using MVsof the control points located at two or three corners of the currentblock through an affine parameter {ρ, θ, c, f } based affine motioninformation prediction. The method mentioned above can also be appliedto the affine parameter based affine motion information prediction,where a set of pre-defined delta values around the base predictor'saffine model parameters can be applied to derive the actual affine modelthat is used in the current block. Because the number of delta valuesare limited, the proposed method can be regarded as a quantized versionof signaling affine parameters.

Following discussion provides some embodiments to specify values of ρand θ to define the affine model. idx_ρ and idx_θ are indices associatedwith the two parameters ρ and θ respectively. When idx_ρ and idx_θ arezero, the model goes back to translational model. When idx)ρ and idx_θare not zero, a small delta can be applied to the base predictor'saffine parameter values in order to generate the affine model parametersfor the current block.

FIG. 15 shows a second flow chart that outlines a process (1500) ofaffine parameter based affine merge with offset. As shown in FIG. 15 ,the process (1500) can start from (S1502) where a merge flag and anaffine_merge_ with offset usage flag of the current block are signaledby the encoder and coded by the decoder subsequently. When both themerge flag and the affine_merge_ with offset usage flag are false, theprocess (1500) proceeds to (S1503) where a traditional merge mode isapplied to predict the current block. When both the merge flag and theaffine_merge_ with offset usage flag are true, the process (1500)proceeds to (S1504). At (S1504), when more than one predictor candidatesare used to decide a base predictor, a base predictor index is signaledby the encoder at (S1506). Otherwise, the process (1500) proceeds to(S1505) where a predefined base predictor index is applied.

The process (1500) then proceeds to (S1507) when the base predictor isdefined either at (S1505) or (S1506). At (S1507), for each of thecontrol points of the current block, a Zero_delta flag is signaled bythe encoder. When the Zero_delta flag is true, the process (1500)proceeds to (S1508) and (S1512) accordingly. At (S1508), current block'sscaling parameter is set to be equal to the base predictor's scalingparameter ρ. At (S1512), current block's rotational parameter is set tobe equal to the base predictor's rotational parameter θ.

When the Zero_delta flag is false, the process (1500) proceeds to(S1509) and (S1511) accordingly. At (S1509), a Delta Scaling Index issignaled by the encoder. The Delta Scaling Index is associated with adelta scaling parameter. At (S1510), the decoder decodes the DeltaScaling Index to derive the delta scaling parameter, and the currentblock's scaling parameter is generated by combining the base predictor'sscaling parameter and the delta scaling parameter. Similarly, at(S1511), A Delta Rotation Index is signaled by the encoder. The DeltaRotation Index is associated with a delta rotational parameter. At(S1513), the decoder decodes the Delta Rotation Index to derive thedelta rotation parameter, and the current block's rotational parameteris generated by combining the base predictor's rotational parameter andthe delta rotational parameter.

The process 1500 subsequently proceeds to (S1514). At (S1514), for eachof the control points of the current block, a Zero_MVD flag is signaledby the encoder. When the Zero_MVD flag is true, the process (1500)proceeds to (S1516) where a translational MV of a control point of thebase predictor is set as a translational MV of a control point of theblock. When the Zero_MVD flag is false, the process (1500) proceeds to(S1515) where a Distance Offset Index and an Offset Direction Index forthe CP of the block are signaled by the encoder. The decodersubsequently decodes the distance offset index to determine a distanceoffset value, and decode the offset direction index to determine anoffset direction. The distance offset value and the offset direction areaccordingly applied onto the translational MV of the control point ofthe base predictor to generate the translational MV of the control pointof the block in the current picture.

In an embodiment of the disclosure, a usage flag is signaled after themerge flag, to indicate whether the proposed method is used or not.Since the proposed method is applied for affine inter prediction, whenthe usage flag is signaled to be true, the affine flag for the currentblock can be inferred to be true.

In another embodiment, a usage flag is signaled after the merge flag andthe affine flag when both the merge flag and the affine flag are true.Otherwise, when the merge flag is false or the affine flag is false, theusage flag is inferred to be false.

When the proposed method is used, a base index can be signaled toindicate which affine predictor candidate is used as the base predictor.When only one affine predictor candidate is allowed, the base index canbe skipped

In an embodiment, the base index indicates which candidate from theaffine merge candidate list to be used as the base predictor.

In another embodiment, the base index indicates which affine MVPcandidate to be used as the base predictor.

In some embodiments, when the affine merge candidate or affine MVPcandidate is unavailable, the proposed method can be disabled or can beenabled with a default base affine model, such as translational model,or zero motion vectors, etc.

For affine parameters, a Zero_Delta flag may be used to indicate whetherthe affine motion parameter delta (AMPD) is zero. When the AMPD isindicated to be zero, the current block's corresponding affine parameteris set as the affine parameter of the base predictor.

In an embodiment, a respective Zero_Delta flag can be explicitlysignaled for each of the affine parameters, such as a rotationparameter, a scaling parameter, and a translational MVD. When therespective Zero_Delta flag is false, the corresponding affine parametercan be signaled.

In another embodiment, only one Zero_Delta flag can be signaled for allthe affine parameters of the block. When the Zero_delta flag is false,all affine parameters can be signaled.

In some embodiments, the delta scaling parameter can be signaled bysending the delta scaling index Idx_ρ. A corresponding delta scalingparameter value can be derived from a delta scaling parameter table thatis associated with the delta scaling index.

In an embodiment, the delta scaling parameter table can be shown inTable 10, where the delta scaling index Idx_ρ can include nine indices,and each of the nice indices can include a corresponding delta scalingparameter Δρ. The delta scaling parameter Δρ can be equal to a valuethat is a multiple of n. The n can be a preset or signaled scalingparameter. The signaling of n can be done at block level, CTU level,slice/picture level or sequence level. For example, n can be 1/16. Thevalue of n can also be a predefined fixed value.

TABLE 10 Mapping of direction IDXs to directions Idx_ρ 0 1 2 3 4 5 6 7 8Δρ 0 +n −n +2n −2n +4n −4n +8n −8n

The delta rotational parameter can be signaled by sending the deltarotation index Idx_θ. The corresponding delta rotational parameter valuecan be derived from a delta rotational parameter table by using theindex Idx_θ.

In an embodiment, the delta rotational parameter table can be shown atTable 11. As shown in Table 11, the delta rotation index Idx_θ caninclude nine indices, and each of the indices maps to a pair of sin andcos value of the corresponding angle of rotation θ.

TABLE 11 Mapping of idx_θ to sin θ and (cos θ){circumflex over ( )}2Idx_θ 0 1 2 3 4 5 6 7 8 Δ(cosθ){circumflex over ( )}2 1 1 1/32 1 1/32 11/16 1 1/16 1⅛ 1⅛ 1¼ 1¼ Δsinθ 0 Sqrt( 1/32) −Sqrt( 1/32) ¼ −¼ Sqrt(⅛)−Sqrt(⅛) ½ −½

In another embodiment, α is a preset or signaled delta angle parameter.Each of the indices can be mapped to a respective delta angel value Δθ,which can be shown in Table 12.

TABLE 12 Mapping of the idx_θ and Δθ Idx_θ 0 1 2 3 4 5 6 7 8 Δθ 0 +α −α+2α −2α +3α −3α +4α −4α

Table 13 provides another mapping example of idx_θ and Δθ.

TABLE 13 Mapping of the idx_θ and Δθ Idx_θ 0 1 2 3 4 5 6 7 8 Δθ 0 +α −α+2α −2α +4α −4α +8α −8α

The signaling of α can be done at block level, CTU level, slice/picturelevel or sequence level. The α can also be a predefined fixed value.

It should be mentioned that above tables are merely examples, and thepossible number of delta values is not fixed to be 8. Other suitablevalues, such as 4, 16, etc. can be used.

In the above examples, the binarization of the delta scaling indexand/or delta rotation index can be configured in the following way: 1bit is used to signal if the index is 0 or not. If yes, not additionalbit is needed. If not, in one embodiment, variable length coding, suchas truncated binary, exponential-golomb code, etc, applies to index from1-8. In another embodiment, if not, fix length coding is used to signalindex from 1-8.

For the affine model's translational motion information part {c, f },the prediction method can be as same as the control point motion vectorprediction methods described above. The control-point based affine mergewith offset method described above can be applied to drive thetranslational MV of the affine block. For example, a translationalzero_MVD flag can be used to indicate whether a motion vector differencebetween the predictions translational MV and the current block'stranslation MV exists. When the translational zero_ MVD flag is false, atranslational distance offset index and a translational offset directionindex can be signaled by the encoder. The decoder can decode thedistance offset index to derive a distance offset value, and decode theoffset direction index to determine an offset direction. The distanceoffset value and the offset direction can be applied onto thetranslational motion vector of the base predictor to generate thetranslational motion vector of the block.

Table 14 provides an example of signaling of indices and flags that canbe applied in the method mentioned above.

TABLE 14 An example of signaling of indices and flags Usage Zero_DeltaIdx_θ Zero_delta Idx_ρ Translational Translational Translational Flagrotation scaling zero_MVD Distance Offset flag flag Flag offset IndexDirection Index

Table 15 provides another example of signaling of indices and flags thatcan be applied in the method mentioned above.

TABLE 15 Another example of signaling of indices and flags UsageZero_Delta Idx_θ Idx_ρ Translational Translational Translational Flagflag zero_MVD Distance Offset Flag offset Index Direction Index

FIG. 16 shows a flow chart outlining a process (1600) according to anembodiment of the disclosure. The process (1600) can be used in thereconstruction of a block coded in intra mode, so to generate aprediction block for the block under reconstruction. In variousembodiments, the process (1600) are executed by processing circuitry,such as the processing circuitry in the terminal devices (210), (220),(230) and (240), the processing circuitry that performs functions of thevideo encoder (303), the processing circuitry that performs functions ofthe video decoder (310), the processing circuitry that performsfunctions of the video decoder (410), the processing circuitry thatperforms functions of the intra prediction module (452), the processingcircuitry that performs functions of the video encoder (503), theprocessing circuitry that performs functions of the predictor (535), theprocessing circuitry that performs functions of the intra encoder (622),the processing circuitry that performs functions of the intra decoder(772), and the like. In some embodiments, the process (1600) isimplemented in software instructions, thus when the processing circuitryexecutes the software instructions, the processing circuitry performsthe process (1600). The process starts at (S1601) and proceeds to(S1610).

At (S1610), prediction information of a block in a current picture canbe decoded from a coded video bitstream. The prediction informationincludes a plurality of offset indices for prediction offsets associatedwith an affine model in an inter prediction mode.

At (S1620), parameters of the affine model can be determined based onthe plurality of offset indices. Each of the plurality of the offsetindices includes a respective pre-defined mapping table that includesindexes and corresponding offset values. The parameters of the affinemodel can be used to transform between the block and a reference blockin a reference picture that has been reconstructed. Tables 3-13 showvarious examples of pre-defined mappings of indexes and offset values,and can be used to determine parameters of the affine model.

At (S1630), samples of the block are reconstructed according to theaffine model. In an example, a reference pixel in the reference picturethat corresponds to a pixel in the block is determined according to theaffine model. Further, the pixel in the block is reconstructed accordingto the reference pixel in the reference picture. Then, the processproceeds to (S1699) and terminates.

In the disclosure, the proposed methods can be used separately orcombined in any order. Further, the methods (or embodiments) may beimplemented by processing circuitry (e.g., one or more processors or oneor more integrated circuits). In an example, the one or more processorsexecute a program that is stored in a non-transitory computer-readablemedium.

The techniques described above, can be implemented as computer softwareusing computer-readable instructions and physically stored in one ormore computer-readable media. For example, FIG. 17 shows a computersystem (1700) suitable for implementing certain embodiments of thedisclosed subject matter.

The computer software can be coded using any suitable machine code orcomputer language, that may be subject to assembly, compilation,linking, or like mechanisms to create code comprising instructions thatcan be executed directly, or through interpretation, micro-codeexecution, and the like, by one or more computer central processingunits (CPUs), Graphics Processing Units (GPUs), and the like.

The instructions can be executed on various types of computers orcomponents thereof, including, for example, personal computers, tabletcomputers, servers, smartphones, gaming devices, internet of thingsdevices, and the like.

The components shown in FIG. 17 for computer system (1700) are exemplaryin nature and are not intended to suggest any limitation as to the scopeof use or functionality of the computer software implementingembodiments of the present disclosure. Neither should the configurationof components be interpreted as having any dependency or requirementrelating to any one or combination of components illustrated in theexemplary embodiment of a computer system (1700).

Computer system (1700) may include certain human interface inputdevices. Such a human interface input device may be responsive to inputby one or more human users through, for example, tactile input (such as:keystrokes, swipes, data glove movements), audio input (such as: voice,clapping), visual input (such as: gestures), olfactory input (notdepicted). The human interface devices can also be used to capturecertain media not necessarily directly related to conscious input by ahuman, such as audio (such as: speech, music, ambient sound), images(such as: scanned images, photographic images obtain from a still imagecamera), video (such as two-dimensional video, three-dimensional videoincluding stereoscopic video).

Input human interface devices may include one or more of (only one ofeach depicted): keyboard (1701), mouse (1702), trackpad (1703), touchscreen (1710), data-glove (not shown), joystick (1705), microphone(1706), scanner (1707), camera (1708).

Computer system (1700) may also include certain human interface outputdevices. Such human interface output devices may be stimulating thesenses of one or more human users through, for example, tactile output,sound, light, and smell/taste. Such human interface output devices mayinclude tactile output devices (for example tactile feedback by thetouch-screen (1710), data-glove (not shown), or joystick (1705), butthere can also be tactile feedback devices that do not serve as inputdevices), audio output devices (such as: speakers (1709), headphones(not depicted)), visual output devices (such as screens (1710) toinclude CRT screens, LCD screens, plasma screens, OLED screens, eachwith or without touch-screen input capability, each with or withouttactile feedback capability—some of which may be capable to output twodimensional visual output or more than three dimensional output throughmeans such as stereographic output; virtual-reality glasses (notdepicted), holographic displays and smoke tanks (not depicted)), andprinters (not depicted).

Computer system (1700) can also include human accessible storage devicesand their associated media such as optical media including CD/DVD ROM/RW(1720) with CD/DVD or the like media (1721), thumb-drive (1722),removable hard drive or solid state drive (1723), legacy magnetic mediasuch as tape and floppy disc (not depicted), specialized ROM/ASIC/PLDbased devices such as security dongles (not depicted), and the like.

Those skilled in the art should also understand that term “computerreadable media” as used in connection with the presently disclosedsubject matter does not encompass transmission media, carrier waves, orother transitory signals.

Computer system (1700) can also include an interface to one or morecommunication networks. Networks can for example be wireless, wireline,optical. Networks can further be local, wide-area, metropolitan,vehicular and industrial, real-time, delay-tolerant, and so on. Examplesof networks include local area networks such as Ethernet, wireless LANs,cellular networks to include GSM, 3G, 4G, 5G, LTE and the like, TVwireline or wireless wide area digital networks to include cable TV,satellite TV, and terrestrial broadcast TV, vehicular and industrial toinclude CANBus, and so forth. Certain networks commonly require externalnetwork interface adapters that attached to certain general purpose dataports or peripheral buses (1749) (such as, for example USB ports of thecomputer system (1700)); others are commonly integrated into the core ofthe computer system (1700) by attachment to a system bus as describedbelow (for example Ethernet interface into a PC computer system orcellular network interface into a smartphone computer system). Using anyof these networks, computer system (1700) can communicate with otherentities. Such communication can be uni-directional, receive only (forexample, broadcast TV), uni-directional send-only (for example CANbus tocertain CANbus devices), or bi-directional, for example to othercomputer systems using local or wide area digital networks. Certainprotocols and protocol stacks can be used on each of those networks andnetwork interfaces as described above.

Aforementioned human interface devices, human-accessible storagedevices, and network interfaces can be attached to a core (1740) of thecomputer system (1700).

The core (1740) can include one or more Central Processing Units (CPU)(1741), Graphics Processing Units (GPU) (1742), specialized programmableprocessing units in the form of Field Programmable Gate Areas (FPGA)(1743), hardware accelerators for certain tasks (1744), and so forth.These devices, along with Read-only memory (ROM) (1745), Random-accessmemory (1746), internal mass storage such as internal non-useraccessible hard drives, SSDs, and the like (1747), may be connectedthrough a system bus (1748). In some computer systems, the system bus(1748) can be accessible in the form of one or more physical plugs toenable extensions by additional CPUs, GPU, and the like. The peripheraldevices can be attached either directly to the core's system bus (1748),or through a peripheral bus (1749). Architectures for a peripheral businclude PCI, USB, and the like.

CPUs (1741), GPUs (1742), FPGAs (1743), and accelerators (1744) canexecute certain instructions that, in combination, can make up theaforementioned computer code. That computer code can be stored in ROM(1745) or RAM (1746). Transitional data can be also be stored in RAM(1746), whereas permanent data can be stored for example, in theinternal mass storage (1747). Fast storage and retrieve to any of thememory devices can be enabled through the use of cache memory, that canbe closely associated with one or more CPU (1741), GPU (1742), massstorage (1747), ROM (1745), RAM (1746), and the like.

The computer readable media can have computer code thereon forperforming various computer-implemented operations. The media andcomputer code can be those specially designed and constructed for thepurposes of the present disclosure, or they can be of the kind wellknown and available to those having skill in the computer software arts.

As an example and not by way of limitation, the computer system havingarchitecture (1700), and specifically the core (1740) can providefunctionality as a result of processor(s) (including CPUs, GPUs, FPGA,accelerators, and the like) executing software embodied in one or moretangible, computer-readable media. Such computer-readable media can bemedia associated with user-accessible mass storage as introduced above,as well as certain storage of the core (1740) that are of non-transitorynature, such as core-internal mass storage (1747) or ROM (1745). Thesoftware implementing various embodiments of the present disclosure canbe stored in such devices and executed by core (1740). Acomputer-readable medium can include one or more memory devices orchips, according to particular needs. The software can cause the core(1740) and specifically the processors therein (including CPU, GPU,FPGA, and the like) to execute particular processes or particular partsof particular processes described herein, including defining datastructures stored in RAM (1746) and modifying such data structuresaccording to the processes defined by the software. In addition or as analternative, the computer system can provide functionality as a resultof logic hardwired or otherwise embodied in a circuit (for example:accelerator (1744)), which can operate in place of or together withsoftware to execute particular processes or particular parts ofparticular processes described herein. Reference to software canencompass logic, and vice versa, where appropriate. Reference to acomputer-readable media can encompass a circuit (such as an integratedcircuit (IC)) storing software for execution, a circuit embodying logicfor execution, or both, where appropriate. The present disclosureencompasses any suitable combination of hardware and software.

Appendix A: Acronyms

JEM: joint exploration modelVVC: versatile video codingBMS: benchmark set

MV: Motion Vector HEVC: High Efficiency Video Coding SEI: SupplementaryEnhancement Information VUI: Video Usability Information GOPs: Groups ofPictures TUs: Transform Units, PUs: Prediction Units CTUs: Coding TreeUnits CTBs: Coding Tree Blocks PBs: Prediction Blocks HRD: HypotheticalReference Decoder SNR: Signal Noise Ratio CPUs: Central Processing UnitsGPUs: Graphics Processing Units CRT: Cathode Ray Tube LCD:Liquid-Crystal Display OLED: Organic Light-Emitting Diode CD: CompactDisc DVD: Digital Video Disc ROM: Read-Only Memory RAM: Random AccessMemory ASIC: Application-Specific Integrated Circuit PLD: ProgrammableLogic Device LAN: Local Area Network

GSM: Global System for Mobile communications

LTE: Long-Term Evolution CANBus: Controller Area Network Bus USB:Universal Serial Bus PCI: Peripheral Component Interconnect FPGA: FieldProgrammable Gate Areas

SSD: solid-state drive

IC: Integrated Circuit CU: Coding Unit

While this disclosure has described several exemplary embodiments, thereare alterations, permutations, and various substitute equivalents, whichfall within the scope of the disclosure. It will thus be appreciatedthat those skilled in the art will be able to devise numerous systemsand methods which, although not explicitly shown or described herein,embody the principles of the disclosure and are thus within the spiritand scope thereof.

What is claimed is:
 1. A method for video encoding in an encoder,comprising: determining a base predictor of a block in a currentpicture; determining a plurality of offset indexes in a plurality ofrespective pre-defined mapping tables, the plurality of offset indexesindicating corresponding offset values to be applied to parameters of anaffine model for the base predictor; and generating a coded videobitstream, the coded video bitstream including prediction informationthat indicates the plurality of offset indexes, wherein the pre-definedmapping tables include a first mapping table of distance offset indexesthat are mapped to different pixel distances and a second mapping tableof offset direction indexes that are mapped to different pairs of offsetdirections on an x-axis and a y-axis.
 2. The method of claim 1, whereinthe plurality of offset indexes further comprises at least one of adelta scaling index and a delta rotation index.
 3. The method of claim2, further comprising: determining the base predictor of the block froma predictor candidate list, the predictor candidate list including morethan one predictor candidate, wherein the prediction informationincludes a base predictor index that indicates the base predictor in thepredictor candidate list.
 4. The method of claim 1, wherein thedetermining the plurality of offset indexes comprises: determining afirst distance offset index of a first distance offset value included inthe pre-defined mapping table of the first distance offset index; anddetermining a first offset direction index of a first offset directionincluded in the pre- defined mapping table of the offset directionindex.
 5. The method of claim 4 wherein a motion vector for one of twoor more control points of the block in the current picture is determinedbased on at least the base predictor, the first distance offset value,and the first offset direction by applying the first distance offfsetvalue and the first offset direction to a motion vector of a controlpoint of the base predictor to generate the motion vector for the one ofthe two or more control points of the block in the current picture whichare input as part of the parameters of the affine model for determiningmotion vectors for remaining points with the block of the currentpicture.
 6. The method of claim 5, wherein the first distance offsetvalue and the first offset direction are applied to the motion vector ofthe control point of the base predictor to generate the motion vectorfor the one of the two or more control points of the block in thecurrent picture.
 7. The method of claim 5, wherein the predictioninformation includes a first zero motion vector difference flag thatindicates whether the first distance offset value and the first offsetdirection are applied to a first motion vector of the control point ofthe base predictor on a first inter prediction direction to generate afirst motion vector for the one of the two or more control points of theblock in the current picture on the first inter prediction direction;and the prediction information includes a second zero motion vectordifference flag that indicates whether a second distance offset valueand a second offset direction are applied to a second motion vector ofthe control point of the base predictor on a second inter predictiondirection to generate a second motion vector for the one of the two ofmore control points of the block in the current picture on the secondinter prediction direction.
 8. The method of claim 1, wherein at leastone of the pre-defined mapping tables is adjustable and indicated at oneof a sequence level, a slice level, a tile level, a tile group level,and a block level.
 9. The method of claim 3, wherein the determining theplurality of offset indexes comprises: determining the delta scalingindex based on a delta scaling parameter included in the pre-definedmapping table of the delta scaling index, and determining the deltarotation index based on a delta rotation parameter included in thepre-defined mapping table of the delta rotation index; and a motionvector for one of two or more control points of the block in the currentpicture is based further on at least the delta scaling parameter and thedelta rotation parameter.
 10. The method of claim 9, wherein theprediction information includes a zero delta flag that indicates whetherthe delta scaling parameter is applied to a scaling parameter of thebase predictor to generate the scaling parameter of the block.
 11. Themethod of claim 9, wherein the prediction information includes a zerodelta flag that indicates whether the delta rotation parameter isapplied to a rotation parameter of the base predictor to generate therotation parameter of the block in the current picture.
 12. The methodof claim 9, wherein the prediction information includes a zero motionvector difference flag that indicates whether a distance offset valueand an offset direction are applied to a translational motion vector ofthe base predictor to generate the translational motion vector of theblock.
 13. An apparatus for video encoding, comprising: processingcircuitry configured to: determine a base predictor of a block in acurrent picture; determine a plurality of offset indexes in a pluralityof respective pre-defined mapping tables, the plurality of offsetindexes indicating corresponding offset values to be applied toparameters of an affine model for the base predictor; and generate acoded video bitstream, the coded video bitstream including predictioninformation that indicates the plurality of offset indexes, wherein thepre-defined mapping tables include a first mapping table of distanceoffset indexes that are mapped to different pixel distances and a secondmapping table of offset direction indexes that are mapped to differentpairs of directions on an x-axis and a y-axis.
 14. The apparatus ofclaim 13, wherein processing circuitry is configured to: determine thebase predictor of the block from a predictor candidate list, thepredictor candidate list including more than one predictor candidate,wherein the prediction information includes a base predictor index thatindicates the base predictor in the predictor candidate list.
 15. Theapparatus of claim 13, wherein the plurality of offset indexes includesa first distance offset index and a first offset direction index; andthe processing circuitry is configured to: determine the first distanceoffset index of a first distance offset value included in thepre-defined mapping table of the first distance offset index, anddetermine a first offset direction index of a first offset directionincluded in the pre-defined mapping table of the offset direction index.16. The apparatus of claim 15, wherein a motion vector for one of two ormore control points of the block in the current picture is determinedbased on at least the base predictor, the first distance offset value,and the first offset direction by applying the first distance offsetvalue and the first offset direction onto a motion vector of a controlpoint of the base predictor to generate the motion vector for the one ofthe two or more control points of the block in the current picture whichare input as part of the parameters of the affine model for determiningmotion vectors for remaining points with the block of the currentpicture
 17. The apparatus of claim 16, wherein the first distance offsetvalue and the first offset direction are applied to the motion vector ofthe control point of the base predictor to generate the motion vectorfor the one of the two or more control points of the block in thecurrent picture.
 18. The apparatus of claim 16, wherein the predictioninformation includes a first zero motion vector difference flag thatindicates whether the first distance offset value and the first offsetdirection are applied to a first motion vector of the control point ofthe base predictor on a first inter prediction direction to generate afirst motion vector for the one of the two or more control points of theblock in the current picture on the first inter prediction direction;and the prediction information includes a second zero motion vectordifference flag that indicates whether a second distance offset valueand a second offset direction are applied to a second motion vector ofthe control point of the base predictor on a second inter predictiondirection to generate a second motion vector for the one of the two ormore control points of the block in the current picture on the secondinter prediction direction.
 19. The apparatus of claim 14, wherein theplurality of offset indices further comprises at least a delta scalingindex and a delta rotation index; the processing circuitry is configuredto: determine the delta scaling index based on a delta scaling parameterincluded in the pre-defined mapping table of the delta scaling index,and determine the delta rotation index based on a delta rotationparameter included in the pre-defined mapping table of the deltarotation index; and a motion vector for one of two or more controlpoints of the block in the current picture is based further on at leastthe delta scaling parameter and the delta rotation parameter.
 20. Anon-transitory computer-readable medium storing instructions which whenexecuted by a computer for video encoding cause the computer to performa method comprising: determining a base predictor of a block in acurrent picture; determining a plurality of offset indexes in aplurality of respective pre-defined mapping tables, the plurality ofoffset indexes indicating corresponding offset values to be applied toparameters of an affine model for the base predictor; and generating acoded video bitstream, the coded video bitstream including predictioninformation that indicates the plurality of offset indexes, wherein thepre-defined mapping tables include a first mapping table of distanceoffset indexes that are mapped to different pixel distances and a secondmapping table of offset direction indexes that are mapped to pairs ofoffset directions on an x-axis and a y-axis.